1. Field of The Invention
The present invention is generally related to a position monitoring device for use with a stepper motor and, more particularly, to a position monitoring device and method which determine the position of a stepper motor by monitoring the voltage magnitudes of command signals issued to the phase windings of the stepper motor from an external control device.
2. Description of The Prior Art
Many different types of devices have been employed to monitor the position of the rotor of a stepper motor. These devices have employed at least two distinct philosophical approaches to perform this function. A first approach is to physically attach a device to the rotor of the motor and monitor the movement of that device as the rotor rotates about its central axis. A second approach is to monitor the phase windings of the stepper motor and respond to the magnitude of current or voltage in those phase windings.
U.S. Pat. No. 4,827,203, which issued to Sakano on May 2, 1989, discloses a rotor position detector for a motor which is capable of detecting an absolute rotation position of a rotor shaft for a single revolution of the shaft. The rotational position detector of the Sakano patent also detects, simultaneously with the absolute detection, a rotor magnetic pole rotation position for motor driving control. The absolute rotational position of the rotor shaft in a single revolution of the rotor shaft is detected and a coded signal representative of the detected position is output. The coded signal is converted into a rotor magnetic position signal for motor driving control by a code converting circuit and is provided as an output therefrom.
U.S. Pat. No. 4,791,343, which issued to Ahrendt on Dec. 13, 1988, provides a stepper motor shaft position sensor which differentiates the current provided when the stepper motor is excited by a step command voltage. This differentiated current is, in effect, a signature that is indicative of whether or not the motor shaft has actually taken the commanded step. If the differentiated current falls below a predetermined magnitude within a predetermined interval the motor shaft is determined to have actually taken the commanded step as indicated by a sensor output at a predetermined logic level. Otherwise, the sensor output is at another predetermined logic level indicated that the commanded step has not actually been taken and the motor is in a stalled condition. The sensor output is applied to a utilizing means which provides a continuous verification of the stepper motor shaft position.
U.S. Pat. No. 4,074,179, which issued to Kuo et al on Feb. 14, 1978, discloses a position detection method and apparatus for use in conjunction with stepper motors. The device and method of the Kuo et al patent indicates the position of a stepping motor rotor as it successfully steps and without the need for physically driven encoders or pulse generators. It is characterized by arrangements for sensing the current or the rate of change of current flowing in the phase winding of the stepping motor.
Japanese Patent No. 60-16190, which was filed on July 5, 1983 and laid open on Jan. 26, 1985, discloses a system for detecting the position of a stepping motor. Using voltage signals received from the phase windings of the stepping motor, the disclosed device forms clock signals which correspond to the number of movement steps of a stepping motor. The clock signals are provided for the purpose of detecting the rotational position of a stepping motor. Rotate signals are provided which determine the rotational direction of the stepping motor. The clock signals are obtained from a one-shot pulse generator and produces pulses that are based on two drive signals having phases which are 90 degrees apart. The rotate signals are obtained from a phase judging means which receives two drive signals and detects the drive signal which rises first.
Certain applications require that a stepper motor be monitored in such a way that the position of the rotor of the stepper motor is accurately determined even when the stepper motor rotor has been caused to rotate numerous complete revolutions in both the clockwise and counter-clockwise directions. For example, in one very specific application for which the present invention is suitable, a stepper motor is intended to be used to drive a valve between completely opened and completely closed positions with a requirement that the valve be accurately positioned at intermediate locations between those extremes. If the connection between the stepper motor rotor and the valve seat is such that a large number of rotations of the rotor is needed to cause the valve seat to travel from its completely opened to its completely closed positions, some means must be provided to accurately monitor the number of complete revolutions and partial revolutions of the rotor and not merely the angular position between the rotor and the stepper motor stator. The monitoring means must also be able to maintain an accurate representation of the rotor position even though the stepper motor has experienced numerous clockwise and counter-clockwise movements as the valve seat moves back and forth not only between the extremes of travel but also between intermediate positions.
In one particular example of an application which requires this type of monitoring, a stepper motor is used to drive a valve seat between extreme positions of the valve that require over 40 revolutions of the stepper motor rotor. Therefore, the monitoring means must be capable of determining not only the relative angular position of the rotor with respect to the stepper motor stator but, in addition, the monitoring means must be able to determine the number of complete and partial revolutions that the rotor traveled relative to a predetermined reference position of the valve seat, such as one extreme end of its travel.